libkmod: properly validate file size

libkmod: properly validate file size

In function kmod_elf_new, the file size has to be properly validated against
section offset. Currently, the file size is considered valid based on
ELF header size + section header size * section count. That is not sufficient.

In fact, ELF specifies a section header offset, which doesn't have to be the
size of the ELF header. The supplied test cases even cover this.

The correct test is: section offset + section header size * section count

This patch also verifies that this value won't overflow.  I don't know a way
to crash a tool due to this bug, because later on the offset check would
prevent out-of-bounds access. An overflow would just mean to access a wrong
part in elf->memory. Yet it's a validation error.

Please note: The file size does not have to be validated against the size
of the ELF header again, elf_identify did this already.